1. Field of the Invention
The invention relates to a memory device having non-volatile memory elements and read-out circuits for reading out the items of memory information stored in the memory elements. The memory elements are implemented in particular as fuse memory elements. Furthermore, the invention relates to a method for operating a memory device of this type.
2. Description of the Related Art
So-called fuse memory elements are increasingly being used in integrated circuits. A fuse memory element essentially comprises a metal-metal connection or some other electrically conductive connection, such as polysilicon for example, having a low contact resistance. The metal-metal connection can be interrupted after the actual production process, whereby the contact resistance of the fuse memory element is increased. The fuse memory element can thus assume the programming states “conducting” and “non-conducting”, that is to say it represents either a logic 0 or a logic 1.
Items of information which specify specific properties of a chip can be permanently stored on the chip by means of fuse memory elements. Said items of information relate for example to the identification number of the chip, individual trimming or adjustment settings of internal voltages or a chip-specific configuration. Furthermore, fuse memory elements are used for example for storing the items of memory information of defective memory elements.
The metal-metal connection of a fuse memory element is interrupted as required either by the application of a current or by the action of a laser beam. Fuse memory elements are referred to as electrical fuse memory elements or else laser fuse memory elements depending on the method by means of which their metal-metal connections can be interrupted.
Furthermore, so-called antifuse memory elements exist, in the case of which an electrical connection is not interrupted, rather such a connection is provided after the actual production process for programming purposes. No distinction is made hereinafter between fuse and antifuse memory elements. Instead, the term “fuse memory element” is understood to mean both types of fuse memory elements.
In the German-language technical literature, the terms “Schmelzbrücken” [“fusible links”], “auftrennbare Schmelzbrücken” [“interruptible fusible links”] or “Sicherungen” [“fuses”] are occasionally used for fuse memory elements. However, even in the German-language technical literature, the English term “fuse” is significantly more common. Therefore, the text hereinafter will refer to fuse memory elements.
For read-out and buffer-storage of their programming state, fuse memory elements are connected to circuits specifically designed for this purpose. A circuit of this type comprises a read-out circuit, which measures the fuse resistance and from this determines the memory information stored in the fuse memory element. In this case, generally a high fuse resistance of an interrupted fuse connection represents a logic 1, while a low resistance of an intact fuse connection represents a logic 0. The memory information determined in this way is buffer-stored in a volatile signal memory, for example a latch signal memory or a flip-flop.
It may happen that a semiconductor chip contains more than 1000 fuse memory elements. In such a case, the read-out circuits and signal memories intended for the fuse memory elements can make a significant contribution to the energy consumption of the chip. Particularly in systems designed for wireless applications, such as mobile radio systems for example, it is necessary to reduce the high standby consumption of the fuse circuits.
For this reason, switches which can isolate whole groups of fuse memory elements and the circuits connected thereto from the supply voltage are incorporated into conventional systems. Said switches are opened, that is to say that the fuse circuits connected to the switches are isolated from the supply voltage, as soon as a control unit establishes that the data stored in the relevant fuse memory elements are not required at this point in time.